1. Field of the Invention
The present invention relates to a text mail system which utilizes an integrated services digital network (which will be referred to as the ISDN, hereinafter) for exchange of text messages and a communication terminal device for use in a text mail system and more particularly, to an embodiment of how to realize the confidential operation of a mail box in which the text messages are temporarily stored.
2. Description of the Related Art
Some conventional communication terminal devices connected to a public telephone network have a so-called automatic answering function, that is, when a user of a communication terminal device as a call receiver is not at home, a message from a caller is stored in the terminal device and later the user can read out the message from the terminal device and confirm it at any time.
Such communication terminal devices, as functions associated with the automatic answering function, further have a function of storing a business message of the caller sent in response to an answering message of the call receiver, a function of reproducing the stored business message through user's direct operation, and a function of sending to all callers answering messages previously stored by the user.
There has been demanded a formation of such a so-called text mail system in which a mail box is opened by utilizing message managing functions of a communication terminal device so that a call receiver can confirm a business message stored in the mail box from a specific party, can store an additional message in response to the message left by the specific party in the mail box, and when receiving a further call from the specific party, can send the additional message only to the specific party.
With this type of communication terminal devices, however, since the communication terminal devices are connected to a public telephone network, a call receiver terminal cannot identify a caller terminal and thus when storing text business messages from callers, the call receiver inevitably must store the text business messages from all the callers.
Further, when sending out the answering message from the call receiver, the answering message is sent to all the callers for the similar reason to the above.
In other words, according to the automatic answering function of this sort of communication terminal device, any consideration is not paid at all to the data confidence or security of the mail box and thus it is impossible to form such a text mail system as sufficiently practical.
As its improvement, there has been proposed such a system that, when a call receiver receives a call associated with the aforementioned automatic answering function, a specific calling party sends to the call receiver a PB dial signal containing a code for specifying the caller.
With this system, since the call receiver can specify the caller party by detecting the code sent from the caller at its PB dial signal receiver and thereafter when the call receiver stores the business message or sends an answering message to the specific party, the communication terminal device having the automatic answering function can be used as a highly practical text mail box.
Even this system, however, has been insufficient in the function of the text mail system because the manner to provide the code data for specifying the caller cannot secure its strict data confidence.
Meanwhile, as the ISDN is recently put in practical use, there have been provided communication terminal devices for connection with the ISDN which can have such a text message automatic answering function as mentioned above.
In the case where such a sort of ISDN communication terminal devices are used, one of the communication terminal devices as a call receiver terminal can specify a caller party, at the time of receiving a call, by utilizing such service inherent in the ISDN that address information of the caller terminal is informed from the network.
Thus, it is considered that, when such a text mail system is formed utilizing the automatic answering function of the ISDN communication terminal devices, the aforementioned party specifying function enables the realization of highly confidential operation of the mail box in the system.
However, the caller address information used in the ISDN is determined by the caller side to merely identify the caller and is not determined at the call receiver side in association with a call issued from a specific party.
For such reasons, even though this sort of conventional ISDN communication terminal device can recognize, at the time of receiving a call, the caller on the basis of the caller address information of the call received from the ISDN but cannot determine that the call is associated with a once-received call or not.
Accordingly, in the event where, in a text mail system utilizing this sort of ISDN communication terminals for example, one of a plurality of users commonly using one of the terminals calls another terminal (call receiver), the call receiver only recognizes the caller on the basis of the caller address information of the received call and immediately operates to start sending an additional message to the caller, but the call receiver cannot operate to send respective additional messages to the individual calls of the plurality of common users of the single communication terminal as the caller terminal.
In other words, this sort of system is the same in the point that the additional messages so far stored are all sent out regardless of from whom the call is issued and thus eventually, it is impossible in the current circumstances to form such a text mail system that can secure a high data confidence even when utilizing, in particular, the automatic answering function of this type of conventional ISDN communication terminal devices.